|  | 1.6 | The Transport Layer of the OSI Reference Model
| 1.6.6 | Explain reliability via acknowledgment
Reliable delivery guarantees that a stream of data sent from one machine will be delivered through a data link to another machine without duplication or data loss. Positive acknowledgment with retransmission is one technique that guarantees reliable delivery of data streams. Positive acknowledgment requires a recipient to communicate with the source, sending back an acknowledgment message when it receives data. The sender keeps a record of each data packet it sends and waits for an acknowledgment before sending the next data packet. The sender also starts a timer when it sends a segment, and it retransmits a segment if the timer expires before an acknowledgment arrives. The Figure shows the sender transmitting Data Packets 1, 2, and 3. The receiver acknowledges receipt of the packets by requesting Packet 4. Upon receiving the acknowledgment, the sender sends Packets 4, 5, and 6. If Packet 5 does not arrive at the destination, the receiver acknowledges with a request to resend Segment 5. The sender resends Packet 5 and must receive an acknowledgment to continue with the transmission of Packet 7.
| Lab Activity
| In this lab you will determine what version and IOS your router is currently running and become familiar with the requirements for updating to a newer version. You will check to see how much flash memory the router has and how much of it is currently used by IOS image and how much is free. You will always want to backup your current IOS before upgrading to a newer version. It is a good idea to keep a backup copy of the IOS image file for each router. The process of downloading a new IOS image from Cisco Connection Online (CCO) will be also be reviewed. The TFTP server method of updating your IOS will be covered in this lab. The primary goal of this lab is to get your router updated to IOS 12.0.

| Lab Activity

In this lab you will determine what version and IOS your router is currently running and become familiar with the requirements for updating to a newer version. You will check to see how much flash memory the router has and how much of it is currently used by the IOS image (system code) and how much is free. You will also check the amount of DRAM (Dynamic Random Access Memory).